Generally, it is known that a hollow cathode ion source for in a vacuum chamber has features that its electrode structure is simple, a plasma having relatively high density is produced, and the operation is stable for a long time, thereby obtaining stable ion beams. Various type hollow cathode ion sources have been proposed. For example, Japanese Patent Kokai No. 62-73542 discloses a hollow cathode ion source of a cold cathode type which comprises a cylindrical discharge chamber having major portion formed as a cathode, anodes attached to the cathode electrode via an electrically insulating member, an inlet for a discharge maintaining gas provided on the cylindrical side of the discharge chamber, an ion extraction opening provided on the cathode portion of the opposite side of the cylindrical discharge chamber, and means for cooling the cathode portion of the cylindrical discharge chamber. A sample gas (or metal vapor) is introduced through the gas inlet into the cylindrical discharge chamber and is ionized by a discharge between the anodes and the cathode to produce desired ions. The produced ions are extracted through the ion extraction opening in a direction perpendicular to the axial line of the cathode. By feeding a coolant such as pure water or the like to the cooling means, the sputtering of the cathode material may be accelerated.
In the hollow cathode ion source of this type, however, since the ion extraction opening is provided on the cylindrical cathode portion of the cylindrical discharge chamber, there can not be obtained an ion beam having a circular cross section of a considerably large diameter. Further, the ions are accelerated near the cathode to cause the ion beam having irregular energy to be naturally projected through the extraction opening on the cathode and therefore some drawbacks may be involved when the ion beam is to be used for an analysis.
In Japanese patent application No. 278767/86 we, the inventors, have proposed a hollow cathode ion source in which an ion extraction slit and an opening for introducing a carrier gas are respectively provided on the front and back surfaces of a cylindrical hollow cathode, an extraction electrode is disposed in front of the ion extraction slit, a floating electrode is interposed between the ion extraction slit and the extraction electrode and is provided with a slit aligned with the ion extraction slit, and an anode is interposed between the floating electrode and the extraction electrode and is provided with a slit communicating with the slit of the floating electrode. Argon gas or other carrier gas is introduced through the opening into the cylindrical hollow cathode. The introduced carrier gas is passed through the ion extraction slit and the slits of the floating electrode and the anode, and a discharge is generated in those slits by applying a suitable discharging voltage thereto, thereby forming a plasma of relatively high density in the slits.
The previously proposed hollow cathode ion source has drawbacks that a discharge may readily occur between the extraction electrode and the anode when the degree of vacuum in the cylindrical hollow cathode is insufficient, and thus it is impossible to apply a higher voltage to the extraction portion. In order to improve the degree of vacuum in the cylindrical hollow cathode to be able to apply the higher voltage to the extraction portion, it is necessary to reduce the quantity of the gas to be introduced into the cylindrical hollow cathode. However, if the quantity of the gas to be introduced is reduced, the gas pressure in the cylindrical hollow cathode decreases and then the mean free path of electrons is lengthened. Therefore, the probability that the electrons arrive at the surface of the cylindrical hollow cathode is higher than the probability that the electrons collide with the gas and metal atoms. As a result, the reduction of the gas quantity makes it difficult to maintain the discharge in the cylindrical hollow cathode. Thus, the previously proposed hollow cathode ion source has not provided sufficient gas efficiency.
It is, therefore, an object of this invention to provide a hollow cathode ion source in which the drawbacks of the above-mentioned conventional ion source can be overcome and an ion beam having a circular cross section and a uniform energy can be obtained with better efficiency.
Another object of this invention is to provide a hollow cathode ion source which can maintain a higher discharging voltage and provide a high ionization rate.
Still another object of this invention is to provide a hollow cathode ion source which is provided with multi-stage floating electrode for enhancing a plasma density.
A further object of this invention is to provide a hollow cathode ion source which is provided with means for increasing a sputtering rate and converging the plasma in a discharging path.